Keep in mind that it's not just the cruise line that is a predictor the demographics of the passengers, but the itinerary and the departure and arrival ports as well.
We've sailed on the Summit twice in the last six months. First in the Caribbean round trip from Ft. Lauderdale. This cruise was overrun with seniors, most from Florida, many of whom were chronic complainers and just plain angry people. Just this month on the Ensenada to Hawaii itinerary on the same ship, it was a much more diverse group, and the seniors who were aboard were generally smart, funny, well-traveled, adventurous, and a pleasure to be around.
I think the reasons for the differences are obvious. Alaska is far more adventurous than the Caribbean in the minds of many seniors. After all, it might be chilly, and you might hit a glacier or get eaten by a bear <g>. Plus you have to fly many miles to unfamiliar cities.
I think the more adventurous the itinerary, the more it involves generally long air hops for most people, the more it involves open jaw air tickets (departing and arriving from different places), the more young and young-at-heart people you'll find on board.